The chancel arch, again, demands height, and the more so if it be wide, as in our own day is necessary. The nave arcades are better pointed than round, as are any others carrying any considerable weight. Buttresses remain necessary at the ends of the arcades, and are desirable as a steadiment to the outer walls, particularly where roofs without a direct tie are made use of, and are further useful as permitting the introduction of larger windows than might be safe without them. In all cases, indeed, where roofs or floors are so constructed as to concentrate pressure upon points, it is clear that buttresses are desirable; and when the efficient size cannot be given them without inconvenience or dissight, it is equally clear that the deficiency may be readily compensated by loading them with lofty pinnacles. It is wrong to use buttresses without any object but appearance, but there are numbers of cases where they are of great advantage, besides those in which we know them to be indispensable. If so many of our arched and vaulted buildings in these days were not mere pretences in lath and plaster, we should have more practical experience of the need of the buttress and of the pointed arch. I was once told by the English Commissioner in Scinde that the European engineers had difficulty in making the native builders there believe that any but a pointed arch will stand. Let us now inquire as briefly as may be into the _rationale_ of _ribbed_ vaulting as distinguished from the _arris_ vaulting of the Roman and earlier Romanesque builders. A groined vault does not of absolute necessity demand the use of ribs any more than the plain waggon-head vault. Even the latter was from an early period frequently divided into compartments or bays by transverse ribs, which were useful as a means of giving it rigidity; but in _groined_ vaulting these were of nearly constant use, both for the same reason, and because the vault, being reduced at its springing to so narrow a footing, required this additional strength. The arrises, however, or diagonal lines of intersection, were always left without ribs. Why, then, was the custom changed? For two important reasons. The first was this: that the intersection forms naturally a feeble line, both from the difficulty, particularly with the rough materials usually employed, of making its construction sound; from its forming an arch of greatly increased width without corresponding increase of height: and from its reduction at the springing level to a pin’s point.